The present invention relates to a square chip resistor which includes an insulation substrate and a resistor provided on the insulation substrate and, in particular, to a square chip resistor which prevents generation of any crack on a protective glass surface due to internal stresses to be produced by change in temperatures.
In recent years, in order to enhance the wiring density of a circuit substrate, there has been often used a very small square chip resistor as a resistance element. To produce the square chip resistor, a resistor layer and an electrode layer are formed on an insulation substrate and a protective glass layer is formed in such a manner that it covers the resistor and electrode layers.
In FIG. 1, there is shown a conventional square chip resistor which is disclosed in Unexamined Japanese Patent Publication (Kokai) Hei-3-212901. In FIG. 1, the conventional square chip resistor includes an alumina substrate 101, an upper surface electrode layer 102 and an end face electrode 103 respectively formed of a silver system thick film electrode, a resistor layer 104 formed of a thick film resistor, and a first glass layer 105, a marking glass layer 106 and a second glass layer 107 which respectively serve as protective layers to cover the resistor 104 and are respectively formed of borosilicate lead system glass. Here, the marking glass layer 106 is provided in order that information inherent in a product such as the model number, resistance value, manufacturing number thereof and the like can be marked on it. Also, a Ni plating layer 108 and a Sn-Pb plating layer 109 are applied onto the exposed electrode surfaces by electrolytic plating in order to improve the soldering properties of the exposed electrode surfaces.
In the conventional square chip resistor, the higher the glass layers are located, the smaller coefficients of thermal expansion they have, in order to prevent generation of any crack on the glass surfaces by stresses produced due to differences between the thermal expansion coefficients of the glass layers 105, 106, 107 and that of the alumina substrate 101.
In the above-mentioned Kokai Hei-3-212901, there are used a first glass layer having a softening point of 550 to 570 degrees and a thermal expansion coefficient of 69.times.10.sup.-7 .degree./C. to 75.times.10.sup.-7 .degree./C., a marking glass having a softening point of 550 to 570 degrees and a thermal expansion coefficient of 68.times.10.sup.-7 .degree./C. to 74.times.10.sup.-7 .degree./C., and a second glass layer having a softening point of 580 to 630 degrees and a thermal expansion coefficient of 62.times.10.sup.-7 .degree./C. to 68.times.10.sup.-7 .degree./C.
To form the above-mentioned multi-glass-layer structure, a glass paste is printed and applied and is then dried, after then these operations are repeated, and finally the glass layers are fired, or the printing and firing treatments of the glass pastes are repeated.
However, it has not been easy to obtain such glass as satisfies the above-mentioned softening point and thermal expansion coefficient by adjusting the component ratio of the borosilicate lead glass.
Also, although the protective glass layers of the conventional square chip resistor prevent generation of any crack as mentioned above, in fact, if the stress due to neat exceeds a limit, then a crack can be generated and widened.